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Topic: Why is the Sky Blue (Read 1801 times)

As artists we appreciate our surroundings and environment more. This also means we ask more questions as we are forced to study our subjects before we can paint them. As the sky features so often in our paintings, especially to create mood, it would be good to know why we see the sky in it's various colors.

Properties of Light

To understand the colors of the sky means we need to know a little more about light. As we are artists, and not scientists, we will keep this to the needed minimum technical details.

Light from the sun 'looks' white, but as we know actually contains all the colors of the rainbow. To picture this better we need to stop thinking of the light from the sun as 'one' ray of light, but rather as a 'group' of beams (each color) which together appear white. Each color's beam is traveling at the same speed (299 792 km/sec), this is called the speed of light. Each color has it's own unique frequency or wave length.

Frequency / Wave Length

What on earth does this mean? Think of a wave in the sea. It goes up and down. The wider the gaps between the waves, the longer the wave length. If the waves follow close to each other, then the wave length is shorter. If you were to stand on a jetty and count the amount of waves going past you per second you would have counted the frequency of the waves. This means that waves with a shorter wave length (close together) have a higher frequency. Also waves that have a longer wave length (further apart) have a lower frequency.

Energy

Picture the difference between a stormy sea (high frequency) and a calm sea (low frequency) which one would you say has more energy? Yes, the stormy sea. So the higher the frequency and the shorter the wave length, the more energy the wave has.

Light also travels in waves, called electromagnetic waves, each color having it's own unique wave length, frequency and energy.

Colour spectrum

The main colors of the spectrum are : Red, Orange, Yellow, Green, Blue and Violet (Purple). You will recognize these as the primary and secondary colors on the color wheel. Red has the longest wave length / lowest frequency and lowest energy. Violet (Purple) has the shortest wave length / highest frequency and highest energy. All the other colors are somewhere in between.

Straight Lines

Light moves in a straight line as long as there is nothing in it's way. In outer space there is nothing to obstruct the light waves so they zoom straight towards us at the speed of light, each color vibrating at it's own frequency. If something was to come in it's path, eg., the space shuttle or an asteroid the light waves would bounce off that object or be absorbed by that object. What happens depends on the object. If a wave from the sea hits a rock, it would bounce off. If it hits the beach (sea sand) some would be absorbed into the sand and the rest would flow over.

A similar thing happens when the light waves reach the earth's atmosphere.

The Atmosphere

The atmosphere is made up of many different particles and gasses. Oxygen and nitrogen are two of the gasses. Dust, water and pollen are three of the particles. These particles don't just float off into space as the earth's gravity keeps them suspended. Obviously more particles can be found closer to the earth's surface than higher up as gravity is stronger closer to the surface.

You can think of particles in the atmosphere as rocks and gasses as sea sand in our sea example.

Rayleigh Scattering

A few things then happen when light waves pass through the atmosphere. Lower frequency / energy light waves (like red and orange) simplistically put amble through the atmosphere only hitting particles and gasses here and there. Higher frequency / energy light waves (like violet and blue) hit the particles and gasses more often. As with rocks and sea sand, different things happen when light waves hit particles and gasses. When they hit particles, they scatter like a wave hitting a rock. When they hit gasses, they are first absorbed and then scattered / radiated out again (called Rayleigh Scattering). When these light waves are scattered we see them as visible light. The color of this light depends on the frequency of the light.

Blue not Violet

All this makes sense except that according to the explanation, the sky should be violet as it has the highest frequency. This is true, but there are more blue light waves than violet so the scattered blue waves are more visible than the scattered violet waves.

The Horizon

Why does the sky become lighter towards the horizon ? You are looking through more of the particles and gasses to see the scattered light. Also the scattered light hits more particles and gasses, scattering again and again. Each time the light wave scatters it loses some of it's intensity appearing lighter and lighter.

Glowing Sunsets

When the sun is high above us during the day, the light waves have a shorter path to travel through the atmosphere so it is mainly the higher frequency waves that get scattered. When the sun is setting however, the light waves have a much longer path to travel through the atmosphere to get to your eye. This means that more and more of the light waves get scattered, including those with lower frequencies (red, orange and yellow).

Deep Space

This is also the reason it is black in space. There aren't enough gasses and particles to scatter the light so that we can see it.

Conclusion

Next time you look up at the beautiful sky rather think of it as your personal and permanent fire works display of scattered light rather that this empty blue shell.